The field of invention is molding processes for highly conductive compositions that are particularly useful for intricately molded conductive products, such as for example for fuel cell plates. These processes involve novel injection, and injection/compression molding processes.
These molding compositions can be formed into high definition complex configurations, including configurations, which are particularly suitable for injections molding techniques. For example, they can be molded into thin plate-like specimens (e.g. 60 to 200 thousandths of an inch) having an intricately patterned network of very narrow, relatively smooth, flow passages. Moreover in accordance with the present invention, these labyrinthine plates can be made substantially exclusively by molding, meaning that the need for complex and expensive machining processes is virtually eliminated. Such specimens are used as electrochemical cell bipolar plates. These plates desirably have a bulk conductivity of at least 40, 50, 60, 70, 80, 90 or even 96 S/cm. They also have desirable surface characteristics; heat, temperature, chemical and shrink resistance; strength; and cost.
Conductive polymers have applications in providing alternatives to traditional conductive materials, which often involve greater labor expenses to manufacture into complex parts. In particular, in instances where the demand justifies significant volumes of a product, polymer-molding expenses may prove far more cost effective than comparable machining expenses for other materials. However in the past, it has proved difficult to achieve both a high level of conductivity and desirable molding characteristics. Generally, high-level weight percentages of an appropriate filler in a polymeric matrix are necessary to achieve satisfactory levels of conductivity. However, these high load levels lead to problems with the strength, durability, and moldability of the resulting composition
One area in particular where it would be beneficial to solve the previously mentioned strength, durability, and molding issues is for application in fuel cells. Electrochemical fuel cells have great appeal as a potentially limitless energy source that is clean and environmentally friendly. These fuel cells can, in addition, be constructed at an appropriate scale for small-scale energy consumption, such as household use, or for industrial scale use, and even for commercial power generation. They have portable applications to power small appliances (such as computers or camping equipment), or automobiles and other forms of transportation. Although these different applications involve differences in size, the fundamental construction remains the same for generation of power varying from less than one to a few thousand kilowatts.
Basically, a fuel cell is a galvanic cell in which the chemical energy of a fuel is converted directly into electrical energy by means of an electrochemical process. The fundamental components of the fuel cell are an electrode comprising an anode and a cathode, eletrocatalysts, and an electrolyte. Work has been done in perfecting both liquid and solid electrolyte fuel cells and the present invention may find use in both types of fuel cells.
Solid electrolytes include polymeric membranes, which act as proton exchange membranes typically fueled by hydrogen. These membranes usually comprise a perfluorinated sulphonic acid polymer membrane sandwiched between two catalyzed electrodes that may utilize platinum supported on carbon as an electrocatalyst. Hydrogen fuel cells form a reaction chamber, which consumes hydrogen at the anode. At the cathode, oxygen reacts with protons and electrons at the electrocatalytic sites yielding water as the reaction product. A three-phase interface is formed in the region of the electrode and a delicate balance must be maintained between the electrode, the electrolyte, and the gaseous phases.
Systems involving the use of other electrolytes have been also been studied. These would include alkaline fuel cells, phosphoric acid fuel cell, molten carbonate fuel cells, and solid oxide fuel cells. However, the principles are similar, as are some of the issues in perfecting these products.
A fuel cell reactor may comprise a single-cell or a multi-cell stack. In any case, the cell includes at least two highly conductive flow field plates that serve multiple functions. These plates may function as current collectors that provide electrical continuity between the fuel cell voltage terminals and electrodes. They also provide mechanical support (for example for the membrane/electrode assembly). In addition, these plates act to transport reactants to the electrodes and are essential to establishing the previously mentioned delicate phase balance.
Typically, the fuel cell plates are thin relatively flat plate members that include a highly complex network of interconnecting channels that form the flow field area of the plate. The configuration of these channels is highly developed in order to maintain the proper flow of reactants and to avoid channeling or the formation of stagnant areas, which results in poor fuel cell performance. It is critical that the flow of the reactants is properly managed, and that the electrocatalysts are continuously supplied with precisely the appropriate balance of reactants. Thus, it is essential for the plates to define and maintain clear passages within the highly engineered flow labyrinth. Moreover, in order to assure a satisfactory life, the plates must be able to resist surface corrosion under a variety of conditions. For example, fuel cells may be placed outside and subject to ambient weather. Thus, the cells must be resistant to stress cracking and corrosion at temperature ranging from xe2x88x9240 to 200 degrees Fahrenheit. Further, since the conditions within the cell are corrosive, the cells must also be resistant to chemical attack at these temperatures from various corrosive substances. For example, the plates may be subjected to de-ionized water, methanol, formic acid, formaldehyde, heavy naptha, hydrofluoric acid, tertafluoroethylene, and hexafluoropropylene depending on the fuel cell type. Moreover, the conditions within the fuel cell may lead to elevated temperatures, i.e. from 150 to 200 degrees Fahrenheit, as well as elevated pressures, i.e. from ambient to 30 p.s.i. Corrosive decomposition needs to be avoided since it almost certainly would cause a system failure by changing the flow patterns within the fuel cell.
Past attempts at solving the various requirements for fuel cell plates have included the use of metal and machined graphite plates. The use of metal plates result in higher weight per cell, higher machining costs and possibly corrosion problems. Machined graphite plates solve the weight and corrosion problems but involve high machining cost and result in fragile products, especially when prepared as very thin plates. Some use of graphite/poly(vinylidene fluoride) plates has been made but these have been characterized as being expensive and brittle and having long cycle times.
U.S. Pat. No. 4,197,178 is incorporated herein for its teaching of the working and compositions of electrochemical cells. U.S. Pat. No. 4,301,222 is incorporated herein for its teachings on graphite-based separators for electrochemical cells.
In the past, known conventional bulk molding compounds have been modified to be conductive by the addition of large amounts of conductive filler, such as graphite. During molding it was observed that the liquid resin phase separated from the filler and was exuded from the molding. Further, it was observed that this occurrence tended to cause cracking in molded specimens that were thin. Moreover, bulk conductivity measurements at different locations within the specimen were inconsistent. In accordance with the present invention, it was discovered that compositions could be formulated which solved the foregoing issues. In particular, the formulations involve the use of a resin matrix with high loadings of a conductive filler; various additional additives, such as initiators, mold-release agents, and carbon black; and optionally one or more rheological agents selected from the group comprising group II oxides, alkaline earth oxides, carbodiimides, polyisocyanates, polyethylene and polytetramethylene fluoroethyl. One possible explanation for the mechanism by which the molding agents work, is that they act to build the apparent molecular weight of the prepolymer (e.g. vinyl ester resin or unsaturated polyester resin). Alternatively, these agents may promote flow such as by reducing shear during molding. The use of these rheological agents eliminates phase separation, as well as cracking and inconsistent conductivity measurements. It is anticipated that these problems are a result of the complex configuration of the specimens being molded along with the very high concentrations of conductive filler.
In addition to solving molding and cracking problems it is anticipated that other properties such as the coefficient of thermal expansion, electrical and thermal conductivity, shrink resistance and mechanical properties may be more uniform and/or otherwise improved as a result of the use of the present invention. In addition to the foregoing improvements it was found that a resin composition of the invention demonstrated a higher glass transition temperature and resulted in an improvement in the hot strength of the molded part. Further improvements are also possible by optimizing both gel time and cure time for the prepolymer by controlling initiator type and amount and inhibitor type and amount. Additionally, the compositions can include the use of a low shrink additive is added to the composition which acts to help perfect the surface characteristics of the molded plate made in accordance with the invention. These additives are generally used in the range of 10 to 50 weight percent based on the total weight of the additive and the resin system, i.e. the resin and any monomers. For the purpose of this invention, the term xe2x80x9clow shrink additivexe2x80x9d is used but may encompass additives which are also termed xe2x80x9clow profile additivesxe2x80x9d and help to reduce the roughness of the surface. Resins may have a tendency to shrink during cure which results in surface defects such as sink marks and microscopic irregularities. Other problems include internal voids and cracks, as well as warpage and inability to mold to close tolerances. For molded fuel cell plates, these imperfections inhibit the ability of the resultant product to contact the proton exchange membrane. The xe2x80x9clow profile additivesxe2x80x9d of the present invention help to compensate for shrinkage and improve the surface smoothness. Further, eliminating the shrink problems results in better stacking of the plates and a better overall fuel cell.
The foregoing improvements in specimens molded from these compositions enable the low cost mass production of bipolar plates as an additional embodiment of the invention. These could be used for portable fuel cells, as well as stationary power units.
In accordance with the invention, the following compositions can be used in a new molding process to accomplish injection molding. In particular, the process of the present invention involves using a double auger to convey the highly loaded molding compositions of the present invention to the feed throat of an injection molding apparatus. Alternatively, it has also been found that a single auger, such as is found in an injection molding machine sold by Krauss Maffei with an AZ100 stuffing unit and a 60 mm screw and barrel for thermoset BMC, can be used where the auger is disposed at a angle of between about 30 and 60 degrees to the horizontal plane of the feed throat. More particularly, the angle is about 45 degrees plus or minus 5 degrees. These processes together contrasts to the traditional process using a hydraulic ram to port the molding composition to the feed throat. However, the traditional molding methods and equipment would fail with potentially catastrophic results when the composition would pack out during the molding process. As compared to these traditional methods, it is preferred that a double auger system with a first and larger horizontally oriented screw, which feeds the smaller vertical type auger feeding into the feed throat. Further, the process involves some zoned temperature gradients with a first and second zone in the first screw barrel having a temperature of from about 70 to about 125 degrees F., and more particularly about 70 to about 100 degrees F. A third zone is located at the mold. This zone is maintained at about 275 to about 325 (i.e. 300 F) which is the temperature at which cure is initiated for most of the compositions in accordance with the invention. It is preferable to avoid temperature variations at the mold level. At normal cure rates, the mold time is typically around 10 to 600 seconds, or more usually 30 to 300 seconds or around one or two minutes. The process can be practiced for single or double gate cavity tools, or even for injection/compression processes in which the mold is slightly opened during fill and the mold is shut to compress the shot.